Electrical circuit for communication systems



SepLZB, 1939. P. R. BENNET ELECTRICAL CIRCUIT FOR COMMUNICATION SYSTEMS Filed April 16, 1937 2 Sheets-Sheet 1 Sept. 26, 1939. p BENNET 2,174,456

ELECTRICAL CIRCUIT FOR COMMUNICATION SYSTEMS Filed Apr: Ll 16, 1937 2 Sheets-Sheet 2 Patented 26, 1939 PATENT OFFICE ELECTRICAL CIRCUIT FOR COLIMUNICA- TION SYSTEMS Paul R. Bennet, Philadelphia, Pa., asslgnor to Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application April 16, 1937, Serial No. 137,364

11 Claims.

This invention relates to communication systems. and, more particularly, to systems of the type employing a one-wayamplifier, a plurality of translating devices at different points, and switching means for reversing the connections of the amplifier with respect to the translating devices to, enable two-way communication. Such systems are adapted for use in oflices to provide for inter-oilice communication, in factories, and inother places where it is desired to provide for communication between several points.

The present invention relates specifically to a novel system of this class, employing a master translating device adapted for location at a master station, a plurality of other similarreversible translating devices adapted for location respectively at a plurality of substations, a one-way amplifier which is preferably located at the master station, and switching means at the master station for reversing the connections of the amplifier and for selectively connecting any one or all of the translating devices at the substations to the amplifier. As used herein, the term master station refers to the place from which the system is to be controlled and from which a person may communicate with'any one or all of several persons at other places, while the term substation refers to any one of the several remote places or points at which the said other 'person may be located. The person at the master station has full control of the system and may speak to a person or persons at any or all of the substations and may also listen to any of the said persons at will.

One object of the invention is to provide a novel systemof this type in which there is a minimum loss in sensitivity of the system when i all of the substations are called as compared to the sensitivity of the system when one substation only is called.

A more specific object of the invention is to provide a system of the stated character wherein the translating devices at the substations are connected in a parallel-series arrangement when all of them are connected to the amplifier, so that the efiective impedance of all of the said translating devices is substantially equal to the 1m pedance of any one of them. In this manner, the variation in sensitivity of the system is reduced to a minimum.

Another object of the invention is to provide a novel switching arrangement which enables the selection of any one of the translating devices at the substations and also enables the desired parallel-series connection of such devices.

A further object of the invention is to provide a novel system of the stated character. employing electrodynamic loud speakers as reversible translating devices, thus employing these devices both as loud-speakers and as microphones. Such devices are characterized by having a very low internal impedance, and therefore, by employing these devices, the input and output impedance of the system may be made quite low, thus minimizing the pick-up of undesired extraneous signals, such as those emanating from power lines and other sources of undesired signals.

Other objects and features of the invention will be apparent hereinafter.

In the accompanying drawings:

Fig. 1 is a diagrammatic illustration of one form of the system;

Figs. 2 to Bare illustrations of the various switch positions;

Fig. 7 is a diagrammatic illustration of an alternative embodiment; and

Figs. 8 to 11 are illustrations of the switch positions.

Referring to Fig, 1, the system comprises essentially an amplifier A, a master speaker MS, a plurality of other speakers designated respectively I to 4, and switching devices S1 and S: which are adapted to perform the functions set forth hereinafter. The master speaker MS, together with the switching devices S1 and S2, are

- adapted to be located at a master station and preferably these elements, together with the amplifier A, are built into a single unit. The speakers numbered l to 4 are adapted to be located respectively at different substations, any one or all of which may be called by the master station.

Each of the translating devices employed in the system may be a conventional electrodynamic loud speaker, the internal impedance of which may, for example, be in the neighborhood of 5 ohms. The frequency response of such a speaker is high at the low end of the audio frequency range and decreases at the higher frequencies.

' The audio amplifier A may comprise a multistage vacuum tube amplifier constructedin accordance with conventional practice and, preferably, has a rising frequency response characteristicto compensate for the drooping frequency response has been found that an audio amplifier having a power output of about 1 watt and having an over-all gain of about 55 db. at 400 cycles is satisfactory for the purposes of this invention.

'person or persons at the I, 2, 3 and 4, or

- Switch The switch S1 may comprise a double-pole double-throw switch as illustrated, the p se of this switch being to reverse the connections of the amplifier A with respectto the master, speaker MS and the conductors 5 and 6. As clearly illustrated, the conductor 5 is connected to one terminal of the amplifier input coil, as-well as to one terminal of the amplifier output coil and to one extremity of the voice coil of the master speaker MS. The other terminals ofthe input and output coils are connected respectively to the movable arms of switch S1. tremity of the voice coil of the master speaker is connected to the stationary contacts I and 3 ofswitch S1. stationarybontacts 9 and ID of switch 8;.

When the switch S1 is in the position shown, which may be designated the listen position, the master speaker MS is connected across the output coil of amplifier A, through the switch contact 1, while the conductors 5 and Ii are connected to the input'coil of the amplifier, the conductor 6 being connected to the input coil through the switch contact I0. When the switch S1 is thrown to its other or "talk" position, however, the connections are reversed so that the master speaker MS is connected to the input coil of the amplifier through the switch contact 8, while the conductors 5 and 6 are connected to the output coil through switch contact 9.

It will be seen, therefore, that if one or more of the speakers I to 4 are connected across the conductors 5 and 6, the system provides for two-way communication between the master station and one or more of the substations and, furthermore,

the system is controlled entirely at the master station.

Preferably, the switch S1 is provided with some means for holding it normally in the listen therefore, the switch will be in such position that the person at the master station may listen to a substations; but whenever he wishes to talk to such persons, he may throw'the switch S1 to the "talk position and hold it in that position against the action of its biasing spring.

The purpose of the switch S2 is to enable the selective connection of any one of the speakers the connection of all four of these speakers, to the conductors 5 and 6. As stated above, this switch is also located at the master station and.is under the control of the person at that station. While any suitable form of switch may be employed, the switch S2 pref-' erablytakes the form of a .so-called "wafer since it is found that a switch of this type is very well adaptedfor the purpose of the invention. This switch comprises a stationary insulating plate or ring II carrying circularly arranged contacts and on insulating disk l2 mounted on a rotatable-shaft I3 and carrying semi- The other ex- The conductor 6 is connected to the.

' circular contact segments 5 and k which are inb, c, d and g are adapted to engage only the extending portions 1 of the rotatable segments when one of the extending portions is brought into engagement with one of the shorter contact arms. Since themechanical structure of such a switch is well known in the art, it is unnecessary to illustrate it further.

Adjacent the switch S2, there are providedfive terminals designated respectively a, b, c, d and e which are connected respectively to the switch terminals 41, b, c, d, and e. The terminal c and its-associated switch terminal 0 are connected via conductor I4 to'the switcmterminal g. The conductor 5 is connected to the switch terminal it, while the conductor 6 is connected to the switch terminal I.

The speakers I, 2, 3 and 4, which as stated above, areto be located at difierent substations or remote points, are connected across difierent' pairs of the terminals a, b', etc. as illustrated. The leads connecting these'speakers to the terminals will preferably comprise twisted pairs, as

illustrated, and these leads may be of any suitable length. The terminal a is preferably grounded as shown.

In order to enable a clear understanding of the various positions of the selector switch S2, an arrow has been placed at the center of the switch and it may be considered that the pointed end or head of this arrow indicates various positions of the switch. There are six positions of the switch in all and, with respect to the central arrow, these positions have been indicated about the switch as "off" all I, '2, 3 and 4. In the ofi" position of the switch, none of the speakers I, 2, 3 and 4 is connected to the apparatus. In the "all position of the switch, all four of the speakers are connected in parallel-series relation as will be further described presently. When the switch is in any one of the four positions I, 2, 3 and 4, the speaker bearing the corresponding number is connected to the apparatus.

Bearingin mind that the semi-circular segments 7' and k are insulated from one another, when the switch S2 is in the off position as illustrated, it will be clearly apparent that none of the speakers I, 2, 3 and 4 is connected between the conductors 5 and 6. When the switch is thrown to its other pofitions successively by rotating the switch counterclockwise, the connections are made as set forth in the following table which may be clearly understood by reference to Figs. 2 to 6 showing respecively the all, I, 2, 3 and 4 positions.

Switch Terminal h is position connected to- Terminal 1 is Speaker 2 connected be- 2 bby segment k. c and e by eegmentj. tween h and f. Speaker 1 open circuited. Speakers 3 and 4 short circuited. 3 cbysegmentk. dandeby seg- Speaker 3 connected bement j. tween h and 1. Speakers 1 and 2 open circuited. Speaker 4 short circuited.

4 dbysegmentk. eby segment}. Speaker 4 connected between h and 1. Speakers l, 2, and 3 open circuit-ed.

In the all" position, the speakers are connected in two parallel branch circuits, one including speakers I and 2 in series and the other including speakers 3 and 4 in series. The two branch circuits are connected in parallel relation across the conductors 5 and 6. Since each of the speakers I, 2, 3 and 4 has the same impedance, when these speakers are connected in the parallelseries arrangement, the effective impedance of the entire group will be equal to the impedance of one of the speakers.

Generally speaking, it will be found desirable to .use in the amplifier A an output system employing a pentode amplifier tube although a triode might be employed. In either event, it will be found that for maximum power with minimum distortion, the output impedance is quite critical. Any departure from the optimum impedance will result in both an increase in distortion and diminution of the available power output and power sensitivity. It is, therefore, of considerable importance that the output circuit be arranged as described so that the combined load impedance of the four speakers is the same as that of one speaker, so that the optimum load impedance is employed. In the all position, where less than four speakers are employed, preferably the missing speakers should be replaced by equivalent impedances, or otherwise acompromise as to optimum load impedance results.

It has been found that when all of the speakers are connected in the parallel-series arrangement, there is a loss in sensitivity of about 6 db. compared to the sensitivity when any one of the speakers is connected to the amplifier. This loss is tolerable, however, and; in fact, is scarcely noticeable. On the other hand, any other connection of the several speakers, such as the connection of all of them in parallel relation or in series relation, would result in a loss in sensitivity of 12 db. or more and this loss would not be tolerable.

It will be seen from the above description that the switching arrangement enables the selection of any one of the four speakers I, 2, 3 and 4 and also enables the parallel-series connection of all four of the speakers, so that the effective impedance connected to the amplifier is always constant and equal to the impedance of one of the speakers. Preferably, the switch S2 is provided with a latching mechanism which causes it to latch into its various positions. Such a latching mechanism is commonly employed on switches of this type. If desired, the switch S2 may be arranged for joint or simultaneous operation with the off and on switch S, of the amplifier, as indicated by the broken-line representation.

This joint control of these two switches should,

of course, be so arranged that the switch S is in its off position when the switch S2 is in ofi" position, as illustrated, and the switch S is in on" I positions.

position when the switch S1 is in any of its other It is important to note that the switching arrangement makes possible the use of only five terminals to which the speakers I to 4 are connected, and at the same time enables the selection of any one or all of the speakers as above described. This is an important feature since it simplifies the apparatus and reduces the cost thereof. v

While it is preferred to employ four speakers at the substations or remote points, it may be noted that any one or several of the speakers may be omitted and may be replaced by a short-circuit connection between the terminals to which it is connected although preferably a jumper having an impedance equivalent to that of the speaker should be employed. It should be noted also that a short-circuit connection should not be connected between the terminals a and c or between the terminals and e since either of these shortcircuit" connections would place a short-circuit between the conductors and 6 when the switch is thrown to its all position and this would render the all position'inoperative. Of course, if only one speaker is to be employed, it may be connected to any of the pairs of terminals and the others'may be short-circuited, but, if two or more speakers are to be used, they should be connected in a manner to avoid the use of shortclrcuit connections between terminals (1 and c and between terminals 0 and e.

In actual practice, it is preferable to include all of the apparatus shown in Fig. 1 within the dotand-dash outline in a single unit adapted to be located at the master station. The leads for the various remote speakers may then be simply connected to the terminals 0', b, etc., provided on this unit.

In Figs. '7 to 11, there is illustrated an alternative form of the system wherein the switch S3. replaces the switch S2 of Fig. 1 and the connections of speakers i, 2, 3 and 4 are somewhat modified. Otherwise, the system is sirmlar to that of Fig. 1.

Referring to Fig. 7, it will be noted that in this instance there are only four terminals a, b, c and d corresponding to the similarly designated terminals of Fig. 1. Each of the lines leading to the speakers I, 2, 3 and 4 has one conductor connected to one of the said terminals, while the other conductor of each line is grounded as illustrated. The four terminals a, b, c and d are connected respectively to stationary terminals 0., b, c and d on switch 8:, which terminals correspond to the similarly designated terminalsof the switch S2 in Fig. 1. As in the system of Fig. 1. the conductor 5 is connected to terminal h, while terminals also corresponding to the similarly designated terminals of switch S2 in Fig. 1. In this instance, however, the terminal g of switch S3 is connected to ground as illustrated.

In Fig. 7, the switch Sa'is shown in the all" position, while in Figs. 8' to 11, the switch S3 is shown in the successive positions 1, 2, 3 and 4. In the "all position, the speakers are connected in parallel branch circuits between the conductors 5 and 6, each of the branch circuits including two of the speakers in series; that is to say, the speakers are connected in parallel-series relation described above with reference to Fig. 1. In this instance, however, each of the speakers is grounded to provide a common ground point in the circuit.

e a a... connec s connec er connec ions tion 1 ed toed w- All Unconcandbby canddby Speuers connected nected. .segment segment in parallel-series n. m. arrangement. 1 c, d and f c by scgc, d and Speaker 1 connected by seg-- ment n. by segbetween is and f. ment 1. ment 1. Speaker 2 open circuited. Speakers 3 and 4 short cir- 2 -d andfby b by segdandgby Speaker 2 connected segment ment n. segment between I: and f. m. 1n. Speakers 1 and 3 open circuited.

Speaker 4 short circui 3 a andfby c by sega and g by Speaker 3 connected Segment ment 1:. segment between )0 and f.

m. m. Speaker 1 short circuited. peakers2 and 4 open cir- 4 a and f by d by sega and a by Speaker 4 connected segment .ment 1:. segment between Ia and I. 'm. m. Speaker 1 short circuited. Speakare 2 and 3 open circuited.

When the switch S3 is rotated counterclockwise to its successive positions, the speakers are connected successively between the conductors 5 and 8. The 'connections which are eiIected by the various positions of the switch may be seen from the following table and by reference to Figs. 8 to 11.

Since each of the speakers I, 2, 3 and 4 is grounded at all possibility of any of the speakers picking up and transmitting undesired noises or sounds when such speaker is inactive. In the absence of the grounding of all of the speakers, there is the possibility that an inactive speaker and its associated leads may serve to pick up and transmit undesired noises or sounds due to the capacity of the speaker and its leads .to ground. This is avoided by the grounding of all of the speakers.

Although certain preferred forms of the invention have been illustrated for the purpose of disclosure, it will be understood that the invention is not thus limited butis capable of modification without departing from its scope as defined by the appended claims.

I claim:

1. In a communication system. signal transfer means, four similar translating devices, and manually-operable means for connecting any one of said translating devices, or all four of said devices in two parallel branches each including two devices in series, to said transfer means, whereby thetotal eilective impedance of the connected translating devices is maintained substantially constant independent of the number of devices connected. Y

2. In a communicationgsystem, signal transfer ,means, four similar translating devices, and

, eil'ective impedance of translating devices, or

manually operable means including a single switching device for connecting any one of said all four of said devices in two parallel branches each including two devices in series, to said transfer means, whereby the total the connected translating devices is maintained substantially constant independent of the number of devices connected.

3. In a communication system, signal transfer means, five terminals, four similar translating devices each connected to a diiferent pair oi said terminals, and -manually-operable means connected between said transfer means and said tertimes, this device eliminates allv necting saiddevice minals for connecting any one or all of said translating devices to said transfer means while maintaining-the total effective impedance of the connected translating devices substantially constant independent of the number of devices connected.

4. In a communication system, signal transfer means, five terminals, four similar translating devices each connected terminals, and a manually-operable switch connected between said transfer means and said terminals for connecting any one or all of said translating devices to said transfer means while maintaining the total eil'ective impedance of the connected translating devices substantially constant independent of the number of devices connected.

to a different pair of said 5. In a communication system. i nal transfer devices each connected to a different pair of said terminals, and manually-operable means connected between said transfer means and said terminals for connecting any one of said devices, or all four of said devices in two parallel branches each including two devices in series, to said transfer means, whereby the total eflective impedance of the connected translating devices -is maintained substantially constant independent of the number of devices connected.

6. In a communication system, signal transfer means, flve terminals, four similar translating devices each connected to a different pair of said terminals, and a manually-operable switch connected between said transfer means and said terminals for connecting any one of said devices, or all four of said devices in two parallel branches each including two devices in series, to said transfer means, whereby the total eflective impedance of the connected translating devices is maintained substantially constant independent of the number of devices connect 7. In a communication system, a one-way signal amplifier having input and output circuits of predetermined impedance, a reversible translating device adapted to be located at a master station, a pair of conductors, switching means adapted to be located atthe master station for connecting said device and said conductors to said input and output circuits respectively or vice versa, four reversible translating devices adapted for location at diiferent substations, and switching means adapted for location at said master station for connecting any one of said four devices, or all four of them in two parallel branches of the connected devices is maintained substantially constant independent of the number of devices connected.

8. In a communication system, a one-way signal amplifier having input and output circuits of predetermined impedance, a reversible translating device adapted to be located at a master station, apair of conductors, switching means adapted to be located at the master station for conand said conductors to said input and output circuits respectively or vice versa, flve terminals, four reversible translating devices each connected to a diiferent pair of said terminals, said devices being adapted for location'at different substations, and switching means at said master station connected between said conductors and said terminals for connecting any one of said four devices, or all four of them in two parallel branches each including two devices in series, to said conductors, whereby the total ettranslating devices, or all of said devices in series-parallel relation, to said transfer means, the series-parallel connection of said devices comprising the said groups included in respective parallel branch circuits, the devices of each group being serially connected in a branch circuit, whereby the total efiective impedance of the connected translating, devices remains constant whether one device 'is connected or all of the devices are connected.

10. In a communication system, signal transfer means, a predetermined number of similar impedance devices at least some of which comprise translating devices, saidnumber being divisible into groups each of which contains the same number of impedance devices as there are groups, and manually-operable means for connecting anyone of said impedance devices, or all i of said devices in series-parallel relation, to said transfer means, the series-parallel connection of said impedance devices comprising the said groups included in respective parallel branch circuits, the impedance devices of each group being serially connected in a branch circuit, whereby the total eflective impedance connected to said transfer means remains constant whether one device is connected or all of the devices are connected.

11. In a communication system, a one-way signal amplifier having input and output circuits, a reversible translating device adapted to be located at a master station, a pair of conductors, switching means adapted to be located at the master station for connecting said device and said conductors to said input and output circuits respectively or vice versa, a predetermined number of similar impedance devices at least some of which comprise reversible translating devices adapted for location at diflerent substations, said number being divisible into groups each of which contains the same number '01 impedance devices as there are groups, and switching means adapted for location at said master station for connecting any one of said impedance devices, or all of said devices in series-parallel relation, to said conductors, the series-parallel connection of said impedance devices comprising the said groups included in respective-parallel branch circuits, the impedance devices of each group being serially connected in a branch circuit, whereby the total e fective impedance connected to said amplifier remains constant whether one device is connected or all of the devices are connected.

' PAUL R. BENNET. 

